Tru Oil is a product produced by outdoor brand Birchwood Casey. It is marketed primarily as a gun stock finish, but is also used in many other applications including the refinishing of furniture and other wooden items such guitars, where it has also found a large number of devoted users along with excellent reviews and results.
Birchwood Casey's Tru-Oil product is a mix that is primarily composed of Linseed Oil and Polyurethane as well as proprietary drying agents. Likewise, a large portion of the oil is said to be soy-based. The drying agents allow the oil to dry very quickly. So fast in fact, that when applying it, it will likely begin drying during the application.
It is not quite a true oil finish, nor a polyurethane, but is a product that appears to seek the benefits of both of these types of finishes. Many appear to choose Tru-Oil because of this versatility.
Safety Warning: As with many oil finishes such as Linseed Oil, when exposed to oxygen while balled up on a rag and so on, Tru-Oil will be able to spontaneously combust! Flatten the rag and allow it to air dry properly before disposal. Some choose to safely burn the rags. Whatever the method, proper care must be taken with rags soaked in Tru-Oil, or other oil-based finishes.
Tru-Oil is typically applied thinly either using either a rag, or as Birchwood Casey recommends, rubbing it in with your bare hand. The latter may be preferable as it eliminates the need to take special care with combustible rags after application.
This product can result in a relatively flat appearance after a lower number of coats, a satin appearance after a higher number of coats, and even a semi or high-gloss finish if special care is taken with wet sanding, buffing and so on. Likewise, a higher number of coats can of course be made more dull using buffing methods as well. However, it is still a much more shiny finish compared to oil and this should be expected even with a lower number of coats.
As stated previously, Tru-Oil, while tending to be glossy, is used in many applications with high-quality results. Some examples are below, including on a gun stock as well as a guitar: